1. Field of the Invention
This invention relates to energy storage and water desalination. More particularly, fluid is injected down a well to form a hydraulic fracture. Fluid may be pumped into the fracture under pressure and later produced from the fracture under pressure and used to generate power or flow into a reverse osmosis unit for desalination.
2. Discussion of Related Art
A number of factors including energy security, price volatility, carbon regulation, tax incentives and fears of anthropogenic global warming are driving rapid growth of renewable energy sources. Since liquid fossil fuels are consumed primarily in the transportation industry due to their outstanding energy density (about 45 MJ/liter) and biofuels provide only limited energy gain, the key role for renewable energy sources is to displace fossil fuel consumption in electric power generation. The U.S. presently consumes on the order of 1 TW (1012 Watts) of electric power, so only renewable technologies that can eventually deliver 100's of GW overall are meaningful grid-scale options. Aside from hydroelectric power, which has been operating at essentially full capacity for decades, only solar- and wind-based systems can be considered at this time. Neither of these is cost-competitive today without substantial publicly-funded subsidies, although capital expenditures and operating costs are expected to drop over time, and may eventually reach price-parity with coal- and gas-fired power plants. Of these, wind-powered turbines are the more economical, with a capital expenditure (capex) of about $1.75/watt, and Texas alone has an installed base with a peak production capacity of roughly 2.5 GW.
These two key renewable resources, wind and solar, suffer from intermittency on both daily and seasonal bases, as illustrated in FIG. 1. Neither is therefore suitable for providing base-load power. Output fluctuations also cause grid instability; without dynamic load-leveling means (e.g. smart grid technologies) renewable power sources must now be limited to less than about ten percent of delivered power on a given electric grid. As a result, renewable electric power at the grid level is limited not only by source economics, but also grid stabilization technologies.
Thus, large scale electric energy storage technology is needed in parallel with renewable energy sources. Table 1 enumerates the characteristics of candidate energy storage technologies. The most common electric storage systems in use today are based on some sort of battery technology; leading candidates include lead-acid, lithium ion and vanadium flow batteries. These are generally useful not only for leveling renewables at the source, but also for peak-shifting and improving reliability at the point of use. As of 2008, installations were being purchased by PG&E for residential areas with a rated capacity of 1 MW supply for 5 hours at a price of $2M USD. These were justified by deferring investment in increased transmission capacity (˜⅔) and partly by improved quality of service (˜⅓). This provides a useful scale and price-point for considering alternative storage technologies: 5,000 kw-hr capacity, and $400/kw-hr price.
TABLE 1Energy-Power-Balancerelated related of Electro-η,costcostPlantlyzerCompressorDischarge($/kWh)($/kW)($/kWh)($/kW)($/scfm)EfficiencyLead-acid Batteries (low)175200500.85Lead-acid Batteries (medium)225250500.85Lead-acid Batteries (high)250300500.85Power Quality Batteries100250400.85Advanced Batteries245300400.7Micro-SMES72,00030010,0000.95Mid-SMES (HTS projected)200030015000.95SMES (HTS projected)5003001000.95Flywheels (high-speed)25,00035010000.93Flywheels (low-speed)300280800.9Supercapacitors82,00030010,0000.95Compressed Air Energy,3425500.79Storage (CAES)Compressed Air storage 50517500.7in vessels (CAS)Pumped Hydro1060020.87Hydrogen Fuel Cell/1550050300112.50.59Gas Storage (low)Hydrogen Fuel Cell/15150050600112.50.59Gas Storage (high)Fuel Cell/150050300112.50.59Underground StorageHydrogen engine/1535040300112.50.44Gas Storage
As an applied example, a wind turbine with a rated capacity of 3 MW and typical utilization factor of 0.3 will generate about 22,000 kw-hr per day. If three battery-based storage units described above were devoted to each wind turbine, the capex would more than double, based on $5.25M for a 3 MW wind turbine installation. Plainly, current battery technology is prohibitively expensive for general grid-scale storage, even with reasonable technical improvements and economies of scale.
Leading technologies for grid-scale electric energy storage include pumped hydro and compressed air energy storage (CAES). Pumped hydro uses off-peak electric power to pump water uphill to a reservoir. This requires ready access to large amounts of water and conveniently situated terrain, both of which are in short supply in the region where wind power density is suitable—the great plains of the central US. This technical approach is certainly proven and reliable, and also enjoys excellent round-trip efficiency of ˜87%. Compressed air storage systems depend on availability of abandoned mines or development of deep sub-surface caverns. This is a proven technology that can be sited over about 85% of the continental US and provides reasonable efficiency at ˜80%. Since compression and expansion of air generates large temperature changes, CAES plant to deal with this parasitic energy channel is relatively complex and expensive. The chart in FIG. 2 locates various storage technologies in Power-Energy space, and clearly shows that pumped hydro and CAES stand alone in combining high total energy with high power capability.
Another key application for storage technologies lies in peak shifting, or delivering extra power during short periods of extreme demand. This region is denoted ‘Distributed Resources’ in FIG. 2. Summer afternoon demand peaks related to air conditioning is a prime example. This is simultaneously a period of low productivity for wind turbines, unfortunately. The chart in FIG. 3 shows the estimated capital costs of various candidate technologies for servicing this application.
As noted above, this application is presently getting addressed by a few early adopters like PG&E, based primarily on deferred investment in transmission lines and improved quality of service. Certainly, there is also a marketing advantage based on the “green cachet” of distributed power.
Until such time as pumped hydro and/or CAES are deployed on a massive scale, we note that there is an interesting arbitrage opportunity in storing excess night-time power from wind turbines and reselling it during the peak demand of summer afternoons. Anecdotally, wind farms are said to actually pay grid operators to take their power at night. Wind power specialists, like Green Mountain Energy, sell wind energy at retail for $0.19/kw-hr during the day. Thus, there exists an opportunity to gross roughly $0.20/kw-hr with a twelve hour storage system. This could be quite a profitable enterprise if the storage technology is inexpensive enough. The economics of existing technologies make this a marginal proposition at best in an environment of tight capital markets and demand for high internal rates of return.
In many areas, there is also a shortage of fresh or potable water. One of the prime methods for desalination of water is reverse osmosis. This process requires pressure to overcome the osmotic pressure of salty water and force the water through a semi-permeable membrane. Thus, additional opportunities exist for combining storage technology and desalination technology.